Academic journal article The Science Teacher

Current Taxonomy in Classroom Instruction: How to Teach the New Understanding of Higher-Level Taxonomy

Academic journal article The Science Teacher

Current Taxonomy in Classroom Instruction: How to Teach the New Understanding of Higher-Level Taxonomy

Article excerpt

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The ability to sequence genes has vastly altered our understanding of higher-level relationships among organisms such as those found at the kingdom level. It is important for biology teachers to incorporate these new views and not retain outdated concepts still present in some textbooks. This article provides an overview of our new understanding of higher-level taxonomy, and suggestions for the utilization of current taxonomy in classroom instruction.

The advent of gene sequencing and other discoveries in biology over the past three decades has fundamentally changed our view of the course of evolution and how organisms are related. Evolutionary distances as measured by DNA sequencing can be used to create phylogenetic trees or maps of relatedness and evolution. Although our understanding of these trees is new and occasionally contentious, general agreement has emerged from an evaluation of them. These new views dramatically alter our understanding of early evolution and of organismic relationships at the level of the kingdom.

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Darwin recommended that biological taxonomy be based on evolutionary relationships. Taxonomy in many textbooks is based on the "five kingdoms of life," typically including Animalia, Plantae, Fungi, Protista, and Monera as proposed by Whittaker in 1969. The five kingdoms are often combined with the concept of prokaryote (cells with no nucleus) versus eukaryote (cells with a nucleus). Both of these systems for organizing life have largely been replaced in current scientific understanding with the concept of the three domains, which divides life into Bacteria, Eukarya, and Archaea. In some textbooks, the third domain of Archaea has been grafted on to the five kingdom tree to create a six kingdom tree, but neither a five- or six-kingdom system properly portrays the evolutionary relationships.

What has replaced the familiar five kingdoms? Beginning in the 1970s, genetic studies have given us an entirely new, experimentally grounded perspective on the interrelationships of organisms, culminating in the tree of life based on molecular evidence (Figure 1A, p. 47). We present here a short history of how the five-kingdom model came into popular use, how it has been replaced with the three-domain model (Figure 1B), and how this reappraisal can be used in the classroom.

A brief history of terms

Until the mid-20th century there was little understanding of microbes and their relationships, and taxonomies were based on conjecture (Sapp 2005, 2006; Woese 1994). For example, Linnaeus (1735) organized all living things into merely two kingdoms: Vegetabilia and Animalia. In the 19th century, a widely embraced concept was embodied in Haeckel's (1866) three-kingdom scheme, which encompassed animals, plants, protists, and at the origin of it all, the little-understood monera, which he included in the protist kingdom (Figure 2). In the late 1960s, Whittaker (1969) included fungi in the collection and codified the five kingdoms that are widely taught today.

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The term prokaryote was coined by Chatton (1938), who first proposed organizing life into two great "empires," prokaryotes and eukaryotes (Figure 1B). His plan noted differences among cell structures rather than any proposed evolutionary or taxonomic differences among the groups. Chatton's prescient plan was rediscovered and made more widely known decades later by Stanier and van Neil. They defined the prokaryote as an organism with no nucleus or mitosis, no membrane-bound internal structures, and the presence of a cell wall with a specific mucopeptide (1962). Many scientists questioned this definition of prokaryote, which relies on negative definition (based on the absence of traits). For example, the simple presence or absence of traits is not sufficient reason to lump flying animals such as birds, bats, and insects together; the trait of flight arose several times and does not create an evolutionary taxonomy. …

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